Large numbers of thermally-actuated sprinklers are installed in structures, both old and new every year. These sprinklers, generally installed in the ceiling, are connected to a water supply, and are intended to release the water into the room when the temperature in the room indicates that a fire/conflagration is taking place.
Numerous methods have been used in the past to trigger release of the sprinkler head. For example, low-melting alloys such as solders are used to bond two components together. When heated above the melting temperature of the eutectic alloy, the bond between the two components is released and a control valve is allowed to spring open. This type of actuator is subject to failure as the solder ages and crystallizes, thus weakening the bond.
In some sprinkler valves, a glass tube is nearly filled with a low-temperature boiling liquid and sealed. As the temperature increases the pressure inside the tube becomes great enough to rupture the tube and it fractures, permitting a spring-loaded valve to open. Premature failure may occur if the sprinkler head is subjected to mechanical shock and the glass tube is cracked. False triggering of sprinkler heads sometimes causes damage that is very expensive to repair, and contributes to the cost of fire insurance.
Thermally-actuated fire safety devices must meet a strict set of codes to be acceptable. Actuation temperature varies, typically between 135 to 170° F. (57-77° C.), depending on the requirements of the installation. One example is a Victaulic Guardian sprinkler head specified as 175° C.
Fire safety sprinklers are continually improved as technology becomes more sophisticated. The current invention introduces the use of a shape memory alloy actuator combined with a novel release mechanism to create a product that will meet current and future needs of fire safety sprinkler heads.
Although shape memory alloys have been proposed for valves, including sprinkler valves, such early proposed devices suffer from many of the defects mentioned above, including failure, based on the structure and the manner in which the shape memory alloy is employed. For example, US 2011/0299915 to Crane et al. describes a shape memory alloy (SMA valve. This valve uses a circular SMA component that is expanded, and force-fit to produce friction-based interference hold that can be released by an increase in temperature. The SMA component is Nitinol (polycrystalline nickel titanium).
To date, Nitinol devices for use in valves such as sprinklers have been difficult to construct and commercialize, at least in part because shape memory alloys such as Nitinol do not have a flat stress plateau, and have proven difficult to build with a reliable and accurate activation temperature range. To meet governmental safety standards for sprinklers, the actuation temperature must be within a narrow margin (e.g., of +/−5° C. or less) for an activation temperature. Such a narrow margin is difficult to achieve with most shape memory alloys, including nickel titanium, because of the relationship between stress, strain, and temperature. For example, the sloped stress plateau introduces uncertainty in the transition temperature depending on the stress and strain of the shape memory alloy actuator. In addition, the transition temperature of many shape memory alloys (including Nitinol) is relatively low (e.g., below 100° C.), limiting its use as a fire sprinkler valve.
Described herein are valves, including sprinkler valves, that may address many of the shortcomings of the prior art identified above. For example, the use of a shape memory alloy actuator combined with a novel release mechanism as described herein provides a robust and reliable valve that will meet current and future needs of fire safety sprinkler heads.